The invention relates to a gas friction vacuum pump with at least two differently configured pump stages, each comprising a rotor section and a stator section.
Molecular and turbomolecular vacuum pumps are friction pumps. In molecular pumps, a moving rotor wall and a resting stator wall are configured and spaced apart in such a way that the pulses transferred from the walls to gas molecules disposed between the walls have a preferred direction. Normally, rotor and/or stator walls are provided with thread-like recesses or projections. Turbomolecular vacuum pumps are provided with intermeshed rows of stator and rotor blades, much like a turbine.
Turbomolecular pumps have a relatively low compression (pressure ratio between pressure on the pressure side and the suction side) and a relatively high suction capacity (pumping speed, volume flow per unit of time). Their manufacture and installation is complex and expensive. Moreover, they require a forevacuum pressure of approximately 10.sup.-2 mbar. Molecular pumps are provided with a relatively high compression but their suction capacity is relatively small.
They deliver pressures of up to 10 mbar and more so that the required complexity for the generation of the forevacuum is less than in turbomolecular pumps. It is therefore known to provide gas friction vacuum pumps with differently configured pump stages, with the pump stage on the forevacuum side usually being a molecular pump stage because of the better critical forepressure.